Mind the gap: the delayed recovery of a population of the biological control agent Megamelus scutellaris Berg. (Hemiptera: Delphacidae) on water hyacinth after winter.

Cold winter temperatures significantly affect the biological control effort against water hyacinth, Pontederia ( = Eichhornia) crassipes Mart. (Pontederiaceae), in more temperate regions around the world. The population dynamics of the planthopper Megamelus scutellaris Berg. (Hemiptera: Delphacidae), a newly released biological control agent of water hyacinth, were recorded on the Kubusi River in the Eastern Cape Province (South Africa) over 15 months to determine the population recovery post-winter. Megamelus scutellaris incurred a severe population decline at the onset of winter when the water hyacinth plants became frost damaged. The combined effect of a population bottleneck and low minimum winter temperatures (6.12°C) below the agent's lower developmental threshold (11.46°C) caused a post-winter lag in agent density increase. Subsequently, the maximum agent population density was only reached at the end of the following summer growing season which allowed the water hyacinth population to recover in the absence of any significant biological control immediately post-winter. Supplementary releases of agents from mass-reared cultures at the beginning of the growing season (spring) is suggested as a potential method of reducing the lag-period in field populations in colder areas where natural population recovery of agents is slower.

Tableau input coupled kinetic equilibrium transport (TICKET) model.

TICKET is a general-purpose, multispecies reactive transport model that is based on the tableau structure in MINEQL. The model can be used in solving problems from simple chemical equilibrium calculations for batch systems to complex one-dimensional, reactive transport computations for surface water, groundwater, and water treatment systems. To streamline model input and model formulation, specifications of equilibrium speciation (including homogeneous precipitation, solid solutions, adsorption, and ion exchange) and linear and nonlinear kinetic reactions are defined directly in the tableau. In addition, the burden of accounting for appearing and disappearing solid phases is circumvented by approximating homogeneous precipitation as a solid solution (with an insoluble seed). The solution technique for the model is based on a one-step algorithm and can be applied to both steady-state and fully implicit, time-variable problems. This approach is particularly well-suited in handling chemical speciation-transport problems with fast, nonlinear reaction kinetics and transient chemical intermediates. TICKET model simulations are presented for several test cases to verify the computational scheme. A model application, which examines the effects of sorption and overlying dissolved oxygen concentration on Fe(II) and As(III) oxidation in a sediment column, is also presented to demonstrate the utility of TICKET in examining complex chemical speciation-transport behavior.